1. Field of the Invention
The present invention relates to an engine operated generator and, more particularly, to an engine operated generator which can maintain a smooth action of controlling the number of revolutions of the engine to a target rate in response to a magnitude of load even when the engine fails to produce a predetermined level of power output.
2. Description of the Related Art
Many engine operated generators used generally as alternating current power sources are equipped with an inverter for stabilizing the output frequency. In such a conventional engine operated generator, an alternating current is generated by a generator driven by an engine, temporally converted into a direct current, and converted back by an inverter to an alternating current output at the commercial frequency before being released. The conventional generator system equipped with the inverter allows its output frequency to be hardly dependent on the number of revolutions of the engine and can determine its output through controlling the number of revolutions of the engine in response to a load.
For example, an inverter equipped engine operated generator is disclosed in Japanese Patent Laid-open Publication (Heisei)5-18285 where the load is estimated in accordance with an output current from the inverter and used for controlling a throttle opening of the engine. This permits the output voltage of the generator to maintain at substantially a uniform level regardless of variations of the load.
Also, disclosed in Japanese Patent Laid-open Publication (Heisei)5-146200 is an engine operated generator which can detect its output voltage at the input of an inverter and compare with a predetermined reference voltage to determine the number of revolutions of the engine corresponding to the load.
The inventors have proposed an engine operated generator which is equipped with a converter consisted of a semiconductor rectifier element for rectifying the output current of a generator and arranged to control the number of revolutions of the engine so that the conduction angle of the semiconductor rectifier element is converged toward a target degree which is less than the maximum limit of conduction angle thus to maintain the output voltage of the converter to a desired level (Japanese Patent Laid-open Publication (Heisei)11-308896). As the generator runs with a moderate margin, it can readily respond to an increase in the load within the margin. Also, the output of the generator can be prevented from being affected by a variation in the number of revolutions of the engine.
It is desired to further improve the conventional generator in which the number of revolutions of the engine can be controlled so that the conduction angle (first state angle) of the semiconductor rectifier element is converged at a target degree determined smaller than the maximum level. The engine in the engine operated generator may fail to produce a predetermined level of power output due to degradation with the passage of time. As the target degree of conduction of the semiconductor rectifier element is based on the primary characteristics of the engine, the degradation of the engine with the passage of time will result in the above drawback.
It is now assumed that the number of revolutions of the engine is controlled between 3000 and 5000 rpm and when the engine remains not degraded, the output load of 1000 VA requires 4000 rpm of the number of revolutions of the engine and 75% of the throttle opening. When the engine is degraded and the output load of 1000 VA is required, the target number of revolutions of the engine is set to 4000 rpm. As the engine is degraded, it can hardly run at 4000 rpm of the target number of revolutions with the previous setting of the throttle opening hence failing to produce a desired power output. For increasing the power output, the throttle opening has to be enlarged. If worse, the throttle opening may be required almost 100% thus creating an overloaded state. This type of drawback may be caused by not only the degradation with the passage of time but also the power shortage due to an individual discrepancy in the output of the engine or the power surplus due to an individual discrepancy in the output of the generator.
It is an object of the present invention to provide an engine operated generator which has a generator arranged to generate an actual power output with a comfortable margin in response to a wider range of electrical load for controlling a stable level of the output voltage and an engine arranged to be smoothly controlled in the number of revolutions regardless of engine degradation with the passage of time or a variation in the output of the generator.
The present invention which has the first feature comprises a converter composed of a semiconductor rectifying element for rectifying the output current of a generator driven by an engine, an inverter for converting a direct current received from the converter into an alternating current at a desired frequency, a semiconductor rectifying element driving circuit for controlling the conduction of the semiconductor rectifying element to adjust the output voltage of the converter to a target level, a conduction rate detecting means for detecting the conduction rate of the semiconductor rectifying element, an engine revolution controlling means for controlling the number of revolutions of the engine so that the conduction rate detected by the conduction rate detecting means is converged at a target rate, a means for detecting the throttle opening of the engine, and a modifying means for decreasing the target rate when the throttle opening exceeds a predetermine degree.
The present invention which has the second feature is that the predetermined degree used in the modifying means is determined and set with relation to the number of revolutions of the engine, the modifying means examines whether or not the throttle opening exceeds the predetermined degree in accordance with the throttle opening and the number of revolutions of the engine.
According to the first and second features of the present invention, when the throttle opening exceeds the predetermined degree, it is judged that the engine is overloaded and the target degree of conduction can be decreased correspondingly. As the target degree of conduction is decreased, the engine revolution controlling means drives the engine to run at an increased number of revolutions so that the rate of conduction does not exceed the decreased target. The increased number of revolutions increases the output of the engine. As the engine runs with a generous margin, the throttle can be controlled to reduce its opening.
The present invention which has the third feature is that the modifying means is arranged to decrease the target rate when the throttle opening remains exceeding the predetermined degree. According to the third feature of this invention, momentary increase of the throttle opening does not misjudgment in detecting process for an overload of the generator. Therefore, reliable control can be proceeded.
The present invention which has the fourth feature is that the modifying means is arranged to reset the target rate to an initial setting when the throttle opening has once exceeded the predetermined degree and been returned back to smaller than the predetermined degree. According to the fourth feature of this invention, the engine speed does not be maintained at the high speed than the speed undesirably.
The present invention which has the fifth feature is that the conduction rate is represented by the conduction angle of the semiconductor rectifying element and the target rate is represented by a target angle of the semiconductor rectifying element. According to the fifth feature of this invention, conduction angle of the semiconductor rectifying element which comprises the converter is modified when the generator is overloaded.